This invention relates to an electrochemical apparatus containing a plurality of parallel, elongated electrochemical cells, each cell having an exterior fuel electrode, where a mixture of fresh feed fuel and recirculated spent fuel can contact the fuel electrodes of the cells, the mixture being introduced transversely to the axial length of the cells in a relatively uniform fashion, and where the mixture contains at least water vapor. Reformation of the fresh feed fuel in the mixture takes place at or near the exterior surfaces of the elongated cells.
High temperature, solid oxide electrolyte fuel cell generator apparatus and fuel cell configurations are well known, and taught, for example, in U.S. Pat. Nos. 4,395,468 and 4,490,444 respectively (Isenberg). There, feed fuel, either H.sub.2 +CO, or previously reformed natural gas, is fed into the apparatus at one end and flows parallel to exterior fuel electrode surfaces of elongated fuel cells. Spent fuel is combusted with spent oxidant in a separate chamber and then exits the apparatus.
Other high temperature fuel cell generator apparatus designs are known, where spent fuel is recirculated and aspirated into fresh, preheated feed fuel, in the center of the apparatus, which mixture is fed through the fuel cells, as taught in U.S. Pat. No. 3,718,506 (Fischer et al.). There, spent fuel exiting from the fuel cells, is mixed with fresh feed fuel, such as propane, at a jet-pump nozzle, for flow from one end to the other of the series connected fuel cells. This combination of spent fuel with fresh fuel prevents soot formation within the cells. Additional spent fuel mixes with spent air and flows around and through a catalytic afterburner structure surrounding the fuel cells. Thisi causes complete combustion, which heats the fuel cells, allowing efficient operation at approximately 800.degree. C.
Another generator design to prevent chilling of the fuel cells is taught in U.S. Pat. No. 4,808,491 (Reichner), where a combusted exhaust of spent fuel and spent oxidant provides corner heating in the generator apparatus. There, fresh feed fuel is fed into a manifold at the bottom of the apparatus, the bottom of which manifold may contain reforming catalyst and may be heated by the combusted exhaust. The feed fuel then flows parallel to the exterior fuel electrode surfaces of the elongated fuel cells. The fresh feed fuel is not mixed with any spent gases within the apparatus.
Natural gas (methane plus ethane, propane, butane and nitrogen) may be a likely fuel for many of these fuel cell apparatus. This natural gas must be reformed, that is, converted to carbon monoxide and hydrogen, through the use of a catalyst and excess water vapor, prior to its utilization in the fuel cell. The reforming reaction is endothermic, requiring a supply of heat, and is best performed at temperatures close to 900.degree. C. The heat required for reforming is a significant fraction of the excess heat that results from fuel cell operation.
Without provision of a heat source, such as the exhaust gas stream of U.S. Pat. No. 4,808,491 described previously, if the reformer is located at the base of the fuel cell core, the normal fuel inlet, thermal gradients may be induced along the cell length. Internal reformation of natural gas fuel directly on an inactive, external fuel cell surface, where the fuel is fed into the base of the fuel cell core, and flows parallel to the exterior fuel electrode surfaces, has been taught in U.S Patent No. 4,374,184 (Somers et al.). This process relieved, somewhat, excessive thermal gradients in the generator region. However, the diffusion velocity of reformable fuel, such as natural gas, in the transverse direction to the exterior fuel cell wall is very high; so that reforming of the fuel occurs rapidly along the fuel cell axial length if catalysis at the fuel cell wall is efficient. In this design, all the reformable fuel needed for the full length of the fuel cell could be reformed in the first 1/3 or less of each fuel cell length. Additionally, in the Somers et al. design, a substantial amount of potential fuel cell active length is lost due to the end cell geometry and the temperature drop.
In an effort to solve some of these problems, a system was designed, as taught in U.S. Pat. No. 4,812,373 (Grimble et al.), where separate fuel feed conduits were used exterior and parallel to, and disposed between the fuel cells, to distribute unreformed, fresh feed fuel along the entire exterior active length of the fuel cells. This allows reforming along the entire length of the fuel cells. Here, steam is incorporated into the feed fuel outside of the generator apparatus requiring a separate heater, mixer and tubing. A plurality of feed conduits is also required, all requiring support plates, each conduit being either open ended, in which case fuel is also fed into the base of the apparatus, or closed ended but porous or minutely perforated.
What is needed is an apparatus having optimally distributed fuel introduction along the entire cell, with use of spent fuel which contains water vapor, to add sufficient water vapor to the fresh fuel to allow in-situ reformation by a catalytic reformer material, which is optimally distributed along the cell length. It is one of the main objects of this invention to provide such an apparatus.